The subject invention is an improvement over the prior art plastic blank configurations that are used in making open-mouth containers. It is known that sundry thermoplastic containers can be made by high-speed, solid-phase processes wherein a thermoplastic blank is worked under certain conditions of high pressure at non-melt temperatures into a preform configuration and then reformed by a thermoforming step into a container to produce a desired container shape having advantageous properties heretofore unobtainable by conventional thermoforming processes. Prior to these methods it was well-known that plastic preforms could be injection molded from molten polymers and later thermoformed into thermoplastic containers, as shown, for example, in U.S. Pat. Nos. 3,305,158 and 3,488,805. Admittedly, injection molding techniques have certain limitations, require relatively expensive equipment, and any preform and subsequent container formed therefrom does not have certain advantageous properties as would be found in a container produced by non-melt, solid-phase techniques.
In effect, the apparent shortcomings of injection molding techniques made high-speed, solid-phase methods more attractive. One non-melt method advocated by the Shell Oil Company employs solid-phase forming and is disclosed in U.S. Pat. No. 3,532,786, to Coffman and U.S. Pat. Nos. 3,499,188; 3,546,746 and 3,757,718 to Johnson. Another non-melt method advanced by The Budd Company for solid-state forming is described in U.S. Pat. No. 4,014,970 to Jahnle. Another non-melt process is described and is referred to as a scrapless forming process, the process being advanced by the Dow Chemical Company and described in U.S. Pat. Nos. 3,739,052; 3,947,204; 3,995,763; and 4,005,967 to Ayres, et al. In these prior art processes a plastic blank is cut from sheet stock material, the blank being approximately equal in weight to the finished plastic container, and processed by high pressure techniques to form said desired container. In general, the blanks in the form of squares are cut in line from extruded single or multilayered sheet by slitting the sheet in the direction of extrusion and then shearing the resulting ribbons. In the scrapless forming process the thus-cut squares or blanks are lubricated by conventional means and the lubricated blanks are placed into a heating oven whereby they are heated to a temperature above the softening point yet below the melting point of the polymeric material. For example, polystyrene is heated to temperature between about 230.degree. to about 280.degree. F., whereas high density polyethylene and polypropylene require a narrower range, the temperature being below the crystalline melting point. When the blanks are at the proper forming temperature, they are loaded into a forming machine, the forming machine having a heated circular mold for solid-phase forming the blanks into preforms. The heated circular mold itself has a center portion maintained at a forming temperature while the peripheral portion of the mold is generally below the softening point of the plastic material. Thus, the blanks are preheated to a temperature ranging from just below the melting point to within a softening range of the material and immediately solid-phase formed or forged in a heated mold into a circular preform. In this way orientation is partially built into the finished container depending on a number of factors including the polymer employed, the extent of forging, the pressure, temperature of forging of the preform at this initial stage. The orientation is brought about since a blank of polymeric material is substantially a volume of unoriented polymer which when subjected to external stress undergoes a rearrangement of the crystalline material, generally in the direction of applied stress. It will be appreciated that a high degree of orientation can be obtained if the blank is forged at a forging temperature a little below the melting point yet above the softening point of the blank itself. In accordance with the scrapless forming process a blank is forged in the forming step by forging means provided with cooled lip forming means whereby the high forging pressure applied is sufficient to cause the plastic blank to flow uniformly outwardly and into the lip forming means to cause the peripheral part of the forged blank to be brought rapidly below the softening point of said material while the center portion is maintained at its forming temperature. This process gives rise to the preform formation which is subsequently indexed to a thermoforming station for the next step of making the desired container.
In the above described preform forming step of the scrapless forming process it has been found that the cut edge portion of a conventional blank exhibits a failing that is carried over into the finished product in that during the forging step where the cut edge portion is brought radially outwardly, it manifests itself upon the finished product as an apparent blemish or surface phenomenon in the form of festoons or scalloped markings. This blemish is unpleasing to the eye, may present a structural defect so that a closure could not be properly seated to seal the container and, therefore, would not be acceptable in a commercial sense. Seemingly, the rheological flow pattern is such that during the forging step the peripheral surface which has been sheared or cut is forced towards the lid ring and becomes in part or in total a structural fault in the sidewall and/or upper rim or flange portions of the container. It will be appreciated that the distance to be traveled by a forged plastic blank material is at its greatest at the midpoints from the corners of a given blank and it is along these loci from these midpoints that the apparent structural imperfection would be at its maximum. Lubrication alone has not resolved this problem in that this manifestation is apparently dependent upon direction of molecular orientation to some extent and simply upon, to a greater degree, the roughened or unsmooth surface established during the severing or cutting stage in the blank forming stage. Different cutting means have been used to better sever the sheet in order to remove these blemishes from a finished product. Attempts have been made to cut or sever from a given themoplastic sheet other configurations, such as hexagonal and the like to provide less plug travel distance during the solid-phase forming in order to reduce these blemish marks or invected manifestations. All these attempts have not been too successful in removing or substantially removing the surface imperfections or blemish marks from the finished container.
A method of reducing imperfections associated with edge cracking and the like for acrylic blanks when subjected to compression forces is taught in U.S. Pat. No. 3,562,383 to Ayres. The patentee discloses that in order to avoid edge cracking the acrylic blank should be provided with a continuous groove along the peripheral edge of the blank, the groove assuming a V- or U-shaped configuration in cross section. Although related in some aspects to the patent of Ayres the subject invention differs in resolving a different problem and employing different structural features for the polymeric blank configuration.
The present invention is a result of the discovery that when a blank of thermoplastic material has a particular configuration, especially the outer edge portions of the blank, the aforementioned imperfections are removed. Therefore, a major object of this invention is to provide the art with a container having no blemishes or irregular edge manifestations proximate the open mouth of the container, the blemishes taking on a more or less a scalloped or festooned appearance.
Another object of this invention is to provide the art with eliminating the festooned or scalloped appearance in finished containers, and therefore to render festooned-free plastic container, both monolithic and multilayered types, said containers having been formed by providing cut-out blanks which are generally polygonal in shape, forming circular preforms from said blanks that were polygonal in shape, and thereafter forming said containers from said circular preforms.
Another object of this invention is to provide a uniform container free of any festooning or invection caused by roughened portions of thermoplastic blanks formed by the scrapless forming processes.
Another object of this invention is to provide a container made by high pressure, non-melt temperature scrapless forming process that is essentially free from surface imperfections attendant with uneven, peripheral surfaces of a plastic blank cutout.
Another object of the subject invention is the aspect of providing a specially contoured blank of thermoplastic material that can readily be uniformly formed by solid-state forming techniques into a preform that has virtually no surface imperfections associated with flange and sidewall formation.
These and other objects of the invention will become more readily apparent from review of the specification, claims and a study of the attached drawings.